The hormone, 1.alpha.,25-dihydroxyvitamin D3, is known to be a highly potent regulator of calcium homeostasis in animals, and more recently its activity in cellular differentiation has been established, V. Ostrem, Y. Tanaka, J. Prahl, H. F. DeLuca and N. Ikekawa, Proc. Natl. Acad. Sci. USA, (1987), 84, 2610. Many structural analogs have been prepared and tested and some of these have been found to exhibit an interesting separation of activities in cell differentiation and calcium regulation. This difference in activity may be useful in the treatment of some cancers and osteoporosis, H. Sai, S. Takatsuto, N. Ikekawa, I. Tanaka and H. F. DeLuca, Chem. Pharm. Bull., (1986), 34, 4508.
Recently, a new class of vitamin D analogs has been discovered, the so-called 19-nor-vitamin D compounds, which, as shown by the general structure below, are characterized by the replacement of the ring A-exocyclic methylene group (carbon 19), typical of the vitamin D system, by two hydrogen atoms. ##STR1## The group R in the above structure represents asteroid side chain as it occurs in any of the natural vitamin D compounds, or in synthetic analogs thereof. A specific example of a 19-nor-vitamin D compound is given by structure 20 in Scheme IV herein. Biological testing of such 19-nor-analogs (e.g. compound 20) revealed an activity profile characterized by high potency in inducing differentiation of malignant cells, with very low calcium mobilizing activity. Thus, such compounds are potentially useful as therapeutic agents for the treatment of malignancies. A method of synthesis of 19-nor-vitamin D compounds has been reported by Perlman et al., Tetrahedron Letters 13, 1823 (1990). However, this method, which involves the removal of the C-19-methylene group in an existing vitamin D compound is not well suited for the larger scale preparation of 19-nor analogs.